Recent research has shown the tactile performance of the hand to be critically dependent on neural activity in up to four populations of specialized nerve endings. These nerve endings respond differently to disturbances in the skin surface. This is disclosed in an article by Johansson R S, Vallbo A B. Tactile sensory coding in the glabrous skin of the human hand. Trends in Neuroscience 6 (1983) 27-32. Although the relationships between neural codes from individual mechanoreceptor populations and the detection of surface topography and texture have not been completely established, they are known to involve information from three mechanoreceptor types (SAI--slow adapting type I, FAI--fast adapting type I, and FAII--fast adapting type II). The roles of different receptor populations are discussed in the following two articles, namely, Johnson K O, Phillips J R. Spatial and nonspatial neural mechanisms underlying tactile spatial discrimination. In: von Euler C. Franzen O, Lindblom O, Ottoson D, ed. Somato-sensory Mechanisms. Plenum Press, New York, N.Y. 1984, pp. 237-248; and Connor C E, Hsiao S S, Phillips J R, Johnson K O. Tactile roughness: Neural codes that account for psychological magnitude estimates. J. Neuroscience 10 (1990) 3823-3836. Establishing the thresholds of these mechanoreceptor populations is thus required to define the tactile acuity of a skin site.
A technique has been developed for determining the sensitivity of SAI, FAI and FAII receptor populations at the fingertip using audiometric-like procedures, see our publication Brammer A J, Piercy J E. Measuring vibrotactile perception thresholds at the fingertips of power-tool operators. Proc UK Informal Group Meeting on Human Response to Vibration, Buxton, September 1991, pp. 1-8. An essential feature of the technique is to control contact between the skin and a vibrating probe. A procedure for measuring vibrotactile perception thresholds is then employed that permits the determination of the subject's performance.
Thresholds recorded in populations of power-tool operators exposed to hand-arm vibration have revealed two basic patterns of abnormality. This is discussed in the papers by Brammer A J, Piercy J E. Measuring vibrotactile perception thresholds at the fingertips of power-tool operators. Proc UK Informal Group Meeting on Human Response to Vibration,, Buxton, September 1991, pp. 107 and in Brammer A J, Piercy J E, Nohara S, Nakamura H, Auger P L, Haines A T, Lawrence M, Brubaker R L, van Netten C. Vibrotactile thresholds in operators of vibrating hand-held tools. In: Akada A, Taylor W, Dupuis H, ed. Hand-Arm Vibration. Kyoei Press, Kanazawa Japan 1990, pp. 221-223. One pattern involves threshold shifts of comparable magnitude at all frequencies, which is consistent with damage to the whole nerve. The second pattern involves threshold shifts at frequencies mediated by only one or two mechanoreceptor types, and is thus suggestive of selective damage to nerve fibers or nerve endings. The latter pattern, which occurs frequently among vibration-exposed workers, may hence be distinguished from whole nerve compression (e.g. carpal tunnel syndrome) by measuring mechanoreceptor-specific vibrotactile thresholds.
A problem with known methods and apparatus for measuring a subject response is the accuracy of the measurements and the interpretation thereof. They generally cannot detect and correct improper subject responses.